1. Field of the Invention
The invention relates data storage apparatus and particularly to a servo positioning system therefor.
2. Description of the Prior Art
In data storage apparatus of the type using a stack of rotating disks as the record medium, data is usually recorded in a plurality of concentric data tracks on the surfaces of the disks. The tracks are closely spaced for example at 300 tracks per inch and the data recorded at densities in excess of 5000 bits per inch. In order to access the recorded data, it is necessary to move a record and playback head (or transducer) to the selected track in which the desired information is stored and maintain the head precisely over the centre of the track with a minimum displacement error in the presence of any possible disturbance for the whole time the information is being read or updated. These two functions may be achieved for example by a head positioning servo system such as that described and claimed in co-pending application Ser. No. 681,656 to Commander el al., filed Apr. 29, 1976 data and assigned in common with the present application.
In the system described in co-pending application Ser. No. 681,656, track positioning information is derived from a read only servo head associated with a pre-recorded servo disk included in the stack of disks. Data recording and playback heads ganged for movement with the servo head are associated with data disks which form the remainder of the stack. Associated servo electronic circuits produce a position error signal indicating the radial position of the dat heads relative to the desired data track on the data disks.
Such prior art systems involving a dedicated servo head and a dedicated servo disk, where the data heads were mounted for movement with the servo head, operate on the assumption that any factors which effected a relative change of the absolute position of a servo track or servo head, will result in a substantially corresponding change to the absolute position of the data track or data head. However, it has been found that as track densities increase, this assumption is no longer valid in that certain low frequency disturbances (typically of less that 100 Hz), either singly or in combination, may affect the data head-data track relationship to a different degree than they affect the servo head-servo track relationship such that a differential exists which may vary at some undetermined variable rate. Since the ability of a data head to recover data reliably requires that it should not be off track by more than 10% of its normal width, off track errors of say 0.0005 inches which are tolerable at track densities of 200 tracks per inch are unacceptable at 500 tracks per inch.
It has been found that low frequency disturbances which may result in such a differential response may be produced by a variety of causes. For example, the differential thermal expansion effects between servo head and servo disk and between data head and data disk produced by a change in ambient temperature are found not to be the same. Thus, compensatory movement of the servo head and disk in response to expansion effects does not produce the correct compensatory movement of the data head and disk, resulting in an off-track condition. Other low frequency differential effects may be produced by slight eccentricity or tilt of the disks on the spindle. Transient low frequency vibrations caused by the head actuator may also create bending modes of vibration in the spindle.
An alternative approach which avoids these disadvantages is to dispense with the servo disk altogether and to provide servo position information on each data disk in sectors alternating with sectors of data. This enables the servo position information to always be derived from the disk being accessed and consequently the low frequency disturbances referred to above have no effect. Systems using sectored servo information are described in, for example, U.S. Pat. No. 3,185,972 to Sippel, U.S. Pat. No. 3,593,333 to Oswald and U.S. Pat. No. 3,864,741 to Schwarz. Sectored servo systems themselves suffer however from another disadvantage, namely that the resulting position error signal is limited in bandwidth by the frequency of occurrence of the servo sectors (typically of the order of 2 KHz for a 60 sector disk rotating at 2000 RPM.)
In order to have a closed loop system with sufficient gain at low frequencies to correct position offsets caused by the above mentioned disturbances and also to enable the head to settle on track at the end of an access motion within an acceptable time a relatively high bandwidth of several hundred Hz is required. At the same time, with a sampled system it is essential that the gain of the system be reduced to zero before the sampling frequency 2 KHz is reached. Such a reduction in gain at 2 KHz also has a significant impact on gain and more particularly phase lag at frequencies as low as 300 Hz. Increased phase lag results in a longer settling time and insufficiently quick response to disturbances.
Conventional attempts to increase phase lag compensation have the side effect of increasing high frequency gain and the system may become unstable at frequencies around 1000 Hz because of mechanical resonance problems.